These mods certainly make the 281 more flexible and greatly expands the variety of LFO waveforms that it can produce. Of course, you could always take 5 patch cords and patch the output to a mutliple and then to 2 external attenuators (provided that you have some) and then from each attenuator back to the 281's Attack CV and Decay CV inputs).

ATOM wrote:

camelneck wrote:

That looks great ATOM. I'm also considering adding extra pots to regulate the amount of "Attack CV" and "Decay CV". I also plan to use "switching jacks" for the "Attack CV" and "Decay CV" inputs. This way I can wire them up so that the "output of the EG" is wired to the Attack CV and Decay CV inputs when a patch cord isn't inserted into these jacks. You can then use the pots to reshape the output of the EG from a linear-type envelope output to an exponential-type envelope output.

Did you wire your module up in this manner? If so, how does it work. (Some scope shots would be wonderful!)

Yes. I had wired the module as you described. Works great.
Expo decay
Peak out one slope expo, one normal.

BUT, I don't get as snappy envelopes that I'd like to get
I always have to add constant negative voltages to the CV ins in order to get snappy envelopes..

Hi Pehr,
The 281 is an envelope generator/LFO that produces envelopes/waveforms with linear slopes. If you want "snappy" envelopes, you want an envelope generator that produces an exponential decay instead of a linear decay.

It is possible to alter the output of a Function Generators so it produces a an exponential or logarithmic decay. Simply take the output of the FG and run it through an attenuverter (CV processor) and connect the output of the attenuverter to the decay CV input of the FG. Turning the attenuverter one way will change the output of the FG from a linear decay to an exponential decay, turning the attenuverter the other way will change the output of the FG from a linear to a logarithmic decay.

If the output of the function generator is connected to the "linear CV input of a VCA", one could make the output of the VCA sound more snappier (or more exponential) by applying the output of the FG to the "exponential CV input of the VCA" (provided the VCA has such an input). However, if the FG is also controlling the frequency of the VCF, any potential benefit will be compromised.

By "snappy" I meant "fast". Sorry.
I got it running at audio frequencies now, that's fast enough for me

Thanks for the reply

The 281 produces linear envelopes in AD and ASR mode and linear waveforms in "cycle" mode. Unfortunately, linear envelopes appear to be less snappy than exponential envelopes (as I will explain below). However, if you take the CV output of a Function Generator (FG) and feed it into the attack CV and decay CV inputs of the same FG, you can change the shape of the 281's envelopes from linear to exponential.

An envelope with an exponential decay would "appear to the ear" to close faster (and be more snappier) than an envelope with a linear decay even if both of them are set for the exact same decay time. This is because the rate of decay of an exponential envelope is like a curve, while the rate of decay of a linear envelope is like a straight line.

Consider this example:

Suppose we have an envelope generator (EG) controlling a VCA, which controls the overall volume of the output sound. This EG has a "shape switch" that allows you to select between two types of outputs: 1) a linear envelope and a 2) exponential envelope.

Suppose the EG's decay time is set for "1 second" and the shape switch is set for "linear". At the midway point, or 1/2 second mark, the linear envelope will cause the volume (output of the VCA) to drop by 50%.
Thus, at the midway point of the decay period, the VCA is half closed.

Suppose the EG's decay time remains at "1 second", but the "shape switch" is now changed to "exponential". At the midway point, or 1/2 second mark, the exponential envelope will cause the volume (output of the VCA) to drop by 90%. Thus, at the midway point of the decay period, the VCA is almost completely closed.

Although neither envelope (shape) will completely close the VCA and mute the volume by 100% until the decay period reaches 1 second, the overall volume drops significantly faster when an exponential envelope is used. This is why exponential envelopes appear to be much snappier than linear envelopes.

Pehr wrote:

I got it running at audio frequencies now, that's fast enough for me Thanks for the reply

I'm confused. The original question was about envelopes or was that a mis-print?

The 291 only produces envelopes in the AD and ASR modes. You appear to be talking about "cycle" mode now. In cycle mode, the 281 functions as an oscillator as opposed to an envelope generator. So yes, it can generate waveforms in both the low frequency and (lower) audio frequency range. To generate waveforms in the audio range, you need really short attack and decay times

Just trying to be helpful ...Last edited by camelneck on Sun Feb 01, 2015 5:22 pm; edited 2 times in total

Hello
I've just finished this module with 2 pcbs (so 4 EGs) and it seems to work great.
But I think there's a problem with the "A" and "C" EGs: they don't hold when set to Sustain mode even when they are triggered by a Gate signal. They always decay after a certain time set by the decay pot, so the gate seems not to work.
What I don't understand is that the "B" and "D" EGs respond very well to the three modes including the Sustain mode.

I've checked diode orientation to see if that could be the same problem as Beauty Of Decay had before. But all my diodes are well soldered in the good orientation. Except if the silkscreen on the pcb has an error of course... any idea about that?

All the ICs placement is correct too.

The only thing I'm not sure is that I've wired only one B+ terminal per pcb to simplify the wiring; however it seem those two B+ pads are connected on the pcb.

Also I wonder if the end pulse could trig an external Yusynth ADSR, cause it doesn't on mine. If I set the release to 100% then the ADSR LED stays on permanently, but with any other setting the ADSR doesn't respond to the End pulse output
edit: I've got a very short pulse on the End jacks... seems to short to trig an ADSR... so those outputs are not a problem

re-edit: so after more tests it appears that the End pulse range is totally random, sometimes it's strong specialy with low rate in cycle mode, sometimes it's very weak and doesn't trig my MPS or anything correctly.
Do you think the Sustain mode problem and the End outs are linked? if so what should I look for?

I have to say that I've set the max rate trimmers for cycle to around 1KHz... if that matters.
thanks

A simple test again:
- I patch one gate which is routed to the input of "B", the two EGs set in quadrature mode, and each EG in AR mode.
- "A" is set with attack to max and decay close max
- "B" is set with attack to min and decay to min too

when I retrig the second gate which trigs "B", the "B" starts quickly and fails, at the same time "A" begin its attack rise; but even if "B" has fallen, "A" finishes its decay, but "B" won't sustain during the decay of "A".

That's exactly the opposite as described on Topp's website...
So maybe should I look on the "B" side for a problem?

Sorry for the redundant posts, but I've found my mistake finally:
I've been too ambitious when I've set the trimmers to have a 1KHz rate in cycle mode without any CV on inputs.
That's so simple that I've checked everything else, and scratched my head a lot before trying to readjust the trimmers. So now I've around 10 sec for max attack time and more or less the same for the max decay time and that's fine.
That's strange how it has influenced the sustain mode and the End outs.. I still don't understand why it has reacted like that.

I'm a bit disappointed with the End pulses because they seem to be unable to trig properly my Yusynth ADSR - maybe too short?... but they trig the MPS correctly, so I will use them for percussive modules only.

edit: it's still wrong! after rehousing the module and test it with my midi2cv ACX, one EG per pcb still take the gate in as short trig and doesn't sustain! when the module was out of the cabinet I've tested the sustain with a clock from the random/SH Yusynth module which is very long (maybe 2 or 3 sec) at slow rates and all the 281's EGS were ok in sustain mode.
So maybe the 281 doesn't like the gates from my interface, but that's stange because I drive other ADSR, and even full synths with it without any problem

Hi
I've finally found how to make the two deficient EGs work in sustain mode:
I've changed the 2K7 resistor to 2K38.

However I think I've turned around the real problem and I'm still unable to figure where's the mistake for sure.

I don't understand why the "A" and "B" don't have the same threshold for the gate mode:
"A" threshold is around 3,76V while "B" is around 4,81V... they should be closer as I've used 1% resistors and triple checked everything on my boards.

I hope Josh will answer my PM, and publish the corresponding schematic. I would like to understand.

Hi, I'm sorry for the delay in response here, I don't check in here often enough.
OK well first off, the schematic is 95% equivalent to 2x the v3 schems. The crucial stuff hasn't changed at all, excepting the peak circuit. Secondly the 2 FGs on the board should be identical operationally, but the fact that you're experiencing the same issue with multiple boards is of note. I'm glad you've come up with a workaround, but I'll look into this further. And yes the end pulses are very narrow, can't recall off the top of my head.. Also- they aren't buffered. The pulse is passively differentiated.. In the buchla trigger inputs are designed to accommodate. A couple other things to note; in the buchla, pulses are bananas and the output won't be momentarily shorted to the sleeve of the jack as it is when plugging or unplugging phone jacks, also pulse outs ,can be multed together!

I finished my quad 281 Function Gen some time ago, but I don't know if I've ever posted a photo or anything. I looooove this module. I don't know if mine is perfectly calibrated. Each section does seem slightly different, but everything works great and I couldn't be happier with it.

For one, I love the pulse output. This seems to me to be an End of Attack-stage pulse. Is that what it's supposed to be? Before building I assumed this was End of Cycle-- but in practice it seems more like End of Attack. By using this pulse to trigger additional Functions/envelopes I love being able to set up timed chains of delayed events-- I can imagine this being really useful for bringing a series of delayed reverb or other effects to processed audio signals. I desperately need to make some kind of external input module for this system.

I also really like the voltage inputs on the attack and decay. In practice, so far, it seems like they work more like modulation inputs. Rather than increasing the attack or decay times, it seems like I'm more often able to get modulation on just the attack part of the CV, or just the decay part---depending on which input. This may have more to do with the voltage I'm feeding them. Not sure yet.

It's just great all around and it's inspiring to work with something different from the standard synthesizer tools. That's one general rule that I find very endearing about the Buchla in general.

Enormous thanks to Topp for making this module available to the proletariat, and of course, all props to the man above (DB).

Hi Josh
thanks for your reply.
I guess my trig problem was because my midi2CV interface sends gates at 4,55V (it should be 5V but il's a bit less)...
I have not managed to use properly the end outs, but I've added a 4K7 resistor in series for each output...
So are they proportional to the gate/trig input voltage?
That would explain a lot!

I would like to know if other builders had the same problem with the "B" EGs and sutain mode... anyway I've made a slight change with this 2K38 resistor.

IIRC Buchla gates are about 10V. I don't know if this BOM is for an exact clone or if it compensates for "other systems"._________________We are selling emotions, there are no emotions in a grid. -mwagener

toppobrillo "A couple other things to note; in the buchla, pulses are bananas and the output won't be momentarily shorted to the sleeve of the jack as it is when plugging or unplugging phone jacks"

I just modified with diodes a Plan B M10 to avoid shorting by inserting phone jacks, which appears to be a problem with this module.

How about the Toppo Buchla 281 DFG? Is it necessary to modify the module in any way to make it fully suitable for use in a Eurorack system? Do I need to solder-in any additional diodes? (if yes, what diodes and where precisely?) (actually I was assuming all should be OK, no mods required)

The schematics for the 2812 dual board contain a lot of GND connections, from the pots and jacks. How should these be connected to the PCB? flying wire to the next available socket? or would there be a more elegant way? I would be hesitating to solder them to the face plate (aluminum).

So, V- (and the bar btw) mark negative voltage side of the power connector. Just use a 10 pin and standard ribbon. This also solves the other question, as the socket supplies for up to three ground sockets in the center.

Hi everybody, that's my very first post here! I'm coming a little late with this but I just picked up an unfinished unit from a previous builder who didn't have time to complete it; I wired it all up and the "B" section works perfectly whilst the "A" section won't get into audio range when set in cycling mode. The trimpot seems to work just fine, it simply doesn't get further than a certain threshold which is pretty far from audio levels. Also its LED doesn't light up, but that might be a problem with the LED itself: any advice before I start debugging? Maybe that's a common issue which has a no-brainer solution…!

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